Missing expression syntax or escaping for `if: ! expr` is not detected

The following is not validated (from [github docs](https://docs.github.com/en/actions/using-jobs/using-conditions-to-control-job-execution)):

> You must always use the ${{ }} expression syntax or escape with '', "", or () when the expression starts with !, since ! is reserved notation in YAML format. For example:
>
> ```
> if: ${{ ! startsWith(github.ref, 'refs/tags/') }}
> ```
>
> For more information, see "[Expressions](https://docs.github.com/en/actions/learn-github-actions/expressions)."

Currently, this will not be detected as a bug:
```
if: ! startsWith(github.ref, 'refs/tags/')
```

The runner will abort with:
```
The workflow is not valid. .github/workflows/build.yml: (Line: 42, Col: 42, Idx: 420) - (Line: 42, Col: 43, Idx: 421): While parsing a tag, did not find expected tag URI.
```

It should be linted as an error.

Reproduce the unescaped expression from the issue in .github/workflows/build.yml and compare it with the documented escaped form. Trace the extension's current workflow-validation entry point and existing diagnostics; done means the unescaped form receives a lint error while valid escaped and expression forms remain accepted.

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